Water-dilutable epoxy resin systems have become increasingly important among the cold-curing aqueous coating compositions ("aqueous lacquers"). These two-component (or two-part) systems exhibit outstanding properties in comparison with one-component (or one-part) waterborne coating materials. The following positive properties deserve emphasis: low or zero organic solvent content, absence of fire hazard, little or no odor pollution, ease of processing, low sensitivity to wet substrates, good drying and rapid through-curing, excellent adhesion to the majority of substrates, very good intercoat adhesion, good corrosion protection on metals, and ease of cleaning of equipment directly after use.
In general, suitable binders for water-dilutable epoxy systems are liquid epoxy resins, which may also include emulsifiers, resins diluted with reactive diluents, and solid-resin dispersions. In the case of liquid resins, use is often made of mixtures of epoxy resins based on bisphenol A and F (A/F resin mixtures) in order to reduce the crystallization tendency. If the liquid resin is to be self-emulsifying, then it is necessary to use an emulsifier which advantageously is nonionic (having a hydrophilic-lipophilic balance (HLB) &gt;16) in order to convert the highly hydrophobic epoxy resin (HLB&lt;1) to the aqueous, continuous phase.
To some extent this objective can also be achieved by water-dilutable hardeners which themselves have emulsifying properties. Unmodified A/F resin mixtures are not self-emulsifying and are only water-dilutable in combination with aqueous hardeners, which are described, for example, in EP-A 0 000 605. Such systems provide very good corrosion protection.
Externally emulsified systems are widely known from the prior art (e.g., DE-A 27 26 269, EP-A 0 491 550, DE-A 41 37 590). Thus, for example, A/F liquid resins diluted with reactive diluents are employed in combination with nonionic emulsifiers. These externally emulsified resins, although having fairly good properties overall, give coatings whose corrosion protection effect is poorer than that of the abovementioned systems owing to the content of hydrophilic emulsifiers.
For epoxy resins, specific emulsifiers are known which are obtained by condensation of diglycidyl compounds with polyoxyalkylenediols. These systems have proven themselves, for example, in the production of aqueous epoxy solid-resin dispersions. For instance, U.S. Pat. No. 4,122,067 is a patent describing an emulsifier system which is obtained by condensation of diglycidyl ethers with polyethylene glycols, the ratio of the number of OH groups (OH) to the number of epoxide groups (EP) being from 1:0.5 to 1:0.833. It therefore relates exclusively to OH-functional emulsifiers. EP-A 0 272 595 demonstrates the technical advance of a comparable emulsifier system ((OH):(EP)=1:0.85 to 1:3.5), which requires the use of specific catalysts in the condensation reaction. Here, the OH-functional and EP-functional systems are described.
EP-A 0 000 605 describes analogous systems which have a z(OH):z(EP) ratio of between 1:1.333 and 1:4, but which are used for the hydrophilization of modified polyamines in order to obtain water-dilutable epoxy hardeners.
EP-A 0 618 245 describes an emulsifier system based on condensation products of an aliphatic polyol having a weight-average molar mass (M.sub.w) of from 200 to 20,000 g/mol and an epoxide compound having at least two epoxide groups per molecule and an epoxide group content of from 500 to 10,000 mmol/kg ("epoxide equivalent weight"=molar mass divided by the average number of epoxide groups per molecule, of from 100 to 2000 g/mol), the ratio of the number of OH groups to the number of EP groups z(OH):z(EP) being from 1:3.51 to 1:10, preferably from 1:4 to 1:8, and the epoxide group content of these condensation products being from 100 to 7000 mmol/kg, preferably from 1000 to 4000 mmol/kg (the epoxide equivalent weight is between about 150 g/mol and 8000 g/mol, preferably between 250 g/mol and 1000 g/mol).
In that patent it was found that these compounds of increased epoxide group content, which therefore exhibit epoxide functionality, are outstandingly suited for use as a nonionic emulsifier in water-dilutable epoxy liquid-resin systems.
The water-dilutable liquid resins formulated with this emulsifier system, like all other such resins disclosed to date, have the substantial defect that, in combination with the majority of commercially available amino-functional aqueous hardeners, they do not indicate the end of the pot life. However, the user is accustomed, from the conventional, i.e., solvent-free or solvent-containing systems, to the fact that after blending the two-component system there is a marked increase in the viscosity toward the end of the pot life (processing time), thereby indicating this life in a markedly perceptible manner. By this means the application of a system which has already reacted is avoided.
In the case of aqueous epoxy systems, in contrast, it is possible only with liquid resins in combination with a few aqueous hardeners to observe a rise in viscosity which appears at the correct time prior to the end of the useful life.
It has so far not been possible to indicate the end of the pot life with epoxy solid-resin dispersions. Even with the dispersions which are otherwise outstandingly suitable for cold-curing two-component systems, such as those described in DE-C 36 43 751 and in DE-A 41 28 487, for example, the lack of pot life indication is seen as a defect.